Regenerative sulfur dioxide scrubbing system

ABSTRACT

An aqueous regenerative SO 2  scrubbing system is provided which has a scrubbing loop and a regeneration loop. The scrubbing loop contains a scrubber through which a thiosulfate-rich aqueous solution containing an alkali metal carbonate continuously circulates under sulfite-forming conditions. The sulfute in the spent absorbent solution is converted to thiosulfate in a sulfite conversion zone located in the scrubbing loop but outside the scrubber. The sulfite conversion is effected by means of a reducing agent containing a water soluble alkali metal hydrosulfite as the essential sulfite reducing agent. The regeneration loop serves to convert the incremental portion of thiosulfate formed in the scrubbing loop to H 2  S and an aqueous solution containing the required amounts of alkali metal carbonate and of alkali metal hydrosulfide for the scrubbing loop. By means of a two-step process in the regenerator, the proper ratio of alkali metal hydrosulfide to carbonate for maintaining the system in the correct internal balance of reagents is established, whereby stable continuous operation of the entire system is achieved.

CROSS REFERENCES TO RELATED APPLICATIONS

Related applications, describing and claiming certain subjects matterhereinafter disclosed are (1) an application, Ser. No. 358,786, filedMay 9, 1973, entitled "Treatment of Gases Containing Sulfur Dioxide," tobe issued Feb. 10, 1976 as U.S. Pat. No. 3,937,787; (2) an application,Ser. No. 410,722, filed Oct. 29, 1973, entitled "Treatment of GasesContaining Sulfur Dioxide;" (3) an application entitled "Sulfur DioxideScrubbing System," Ser. No. 581,073, filed May 27, 1975; (4) anapplication entitled "Reduction of Aqueous Thiosulfate Solutions," Ser.No. 583,211, filed May 29, 1975; (5) an application entitled "Method ofControlling the Reduction of Aqueous Thiosulfate Solutions," Ser. No.581,178, filed May 27, 1975, all of these applications assigned to theassignee of the present application; and (6) an application, Ser. No.498,683, filed Aug. 19, 1974, entitled "Removal of Sulfur Dioxide fromGases Containing Sulfur Dioxide and Oxygen," now U.S. Pat. No.3,906,080, assigned to Consolidation Coal Company, a wholly-ownedsubsidiary of Continental Oil Company.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aqueous regenerative SO₂ scrubbingsystems which use a sulfite-forming SO₂ absorbent.

2. Description of the Prior Art

The aqueous regenerative SO₂ scrubbing system of which the presentinvention is an improvement is fully described in the above-relatedapplications. It comprises a scrubbing circuit and a regenerativesection. In the scrubbing circuit, there is a scrubber through which anaqueous scrubbing solution continuously circulates in contact with theSO₂ -containing gas. A substantially constant high concentration, i.e.at least ten percent (10%) by weight of either sodium or potassiumthiosulfate is maintained in the aqueous scrubbing solution throughoutits traverse around the scrubbing circuit. The effective SO₂ absorbentis not thiosulfate but rather is either sodium or potassium carbonatedissolved in the aqueous scrubbing solution. The term "carbonate" asused herein includes both carbonate (CO₃ ⁼ ) and bicarbonate (HCO₃ ⁻ ).The carbonate is converted by reaction with SO₂ to sulfite in thescrubber under sulfite-forming conditions. Similarly, the term "sulfite"as used herein includes both sulfite (SO₃ ⁼ ) and bisulfite (HSO₃ ⁻ ).The letter "M" as used hereinafter means either sodium or potassium.

A second very rapid reaction occurs in the scrubbing circuit butexternal to the scrubber. That reaction is the conversion of sulfite byMHS to thiosulfate (M₂ S₂ O₃) in a sulfite conversion zone, whereby thesulfite concentration in the scrubbing solution is reduced to very lowlevels.

The required carbonate and the required MHS are produced in theregenerative section by reacting the M₂ S₂ O₃ which is withdrawn fromthe scrubbing circuit in a slipstream with a reducing gas containing COat an elevated temperature and pressure. The regeneration reaction maybe expressed as:

    3 M.sub.2 S.sub.2 O.sub.3 + 12 CO + 5 H.sub.2 O → 2 M.sub.2 CO.sub.3 + 2 MHS + 4 H.sub.2 S + 10 CO.sub.2                       1.

however, in reality, there appear to be several side reactions in whichproducts may react with the reactants and with each other. Accordingly,careful control of the conditions under which reaction (1) is conductedis essential to form the required quantities of carbonate and MHS forreturn together in the regenerated aqueous solution to the scrubbingcircuit to repeat the cycle.

The maintenance of the proper ratio of MHS to carbonate in theregenerated solution which is returned to the sulfite conversion zone inthe scrubbing circuit is essential to satisfactory operation of thescrubbing and regenerative cycle. That ratio is maintained provided thefollowing ratio, R, is maintained at or below 1 in the regeneratedsolution, preferably between 0.75 and 0.98. ##EQU1## where: (S^(o) ) =gram atoms sulfur with valence number zero/100 grams solution.

(S.sup.⁻²) = gram atoms sulfur with valence equal to -2/100 gramssolution.

ΣM = gram atoms of M/100 grams solution present in said aqueous effluentas MHS, M₂ S, M₂ S_(x), M₂ CO₃, MHCO₃, and MOH (where M is Na or K).

When R is greater than 1, there will be excess MHS and associatedsulfides in the regenerated solution. Such excess will reach thescrubbing zone of the scrubbing circuit where it is converted to H₂ S byhydrolysis and/or CO₂ stripping. When R is less than 0.75, there isinsufficient MHS and associated sulfides to maintain the sulfiteconcentration of the recirculating solution at the desired predeterminedlow level.

Prior art patents which describe processes similar in some respects tothe above-described process include the following: U.S. Patent Nos.1,937,196, H. A. Gollmar, Nov. 28, 1933; 2,729,543, J. L. Keller, Jan.3, 1956; 3,431,070, J. L. Keller, Mar. 4, 1969; 3,574,097, P. Urban,Apr. 6, 1971; 3,635,820, P. Urban, Jan. 18, 1972; 3,644,087, P. Urban,Feb. 22, 1972; 3,714,338, P. Urban, Jan. 30, 1973; 3,859,416, P. Urban,Jan. 7, 1975.

3. The Problem

The ratio, R, may be established and maintained at or below 1 byconducting the reduction of thiosulfate in a single reduction zone. Thiszone, as will be later described, operates at a temperature and pressurewhich are much higher than those maintained in the scrubbing circuit. Ithas been discovered that the value of R undergoes a significant decreasein the course of the return of the effluent aqueous product from asingle reduction zone to the scrubbing circuit; and may even be of suchmagnitude as to cause the value of R to fall below the desired range of0.75 to 0.98.

Accordingly, the primary object of this invention is to provide for theassured return to the sulfite conversion zone of the above-describedscrubbing circuit of a regenerated aqueous solution having not only thedesired value of the ratio, R, but also a value of R which is stable.

SUMMARY OF THE INVENTION

The improvement of this invention is in the regenerative section of theabove-described wet regenerative SO₂ scrubbing system. The improvementinvolves the addition of a flash decomposition zone to the thiosulfatereduction zone. In the thiosulfate reduction zone, conditions areselected for the reduction of thiosulfate so that the value of theratio, R, for the effluent aqueous solution from said thiosulfatereduction zone shall be greater than 1 and less than 1.5, instead of avalue of 1 or less. The flash decomposition zone is operated underconditions effective to reduce the value of R to a value from 0.75 to1.0 inclusive. The change in conditions which is critically essential tothe reduction in the value of R is reduction in pressure. Because of thereduction in pressure, MHCO₃ (where M is Na or K) is converted to M₂ CO₃through the reaction

    2 MHCO.sub.3 = M.sub.2 CO.sub.3 + H.sub.2 O + CO.sub.2     2.

and MHS is converted to MHCO₃ through the reaction

    MHS + H.sub.2 O + CO.sub.2 = MHCO.sub.3 + H.sub.2 S

water also flashes from the solution; and the latent heat supplied tothe water vapor causes a substantial reduction in the temperature of theliquid.

The objects and advantages of the present invention will become moreapparent upon reference to the following description of the preferredembodiment and to the accompanying drawing in which the preferredembodiment is schematically shown.

PREFERRED EMBODIMENT

The schematic flowsheet of the accompanying drawing represents thepreferred embodiment of the improved process of this invention. Thesulfite-forming agent which is preferred for use in the scrubbing zoneis potassium carbonate because of the high solubility of potassiumthiosulfate in water.

Scrubbing Circuit

Referring to the drawing, an SO₂ -containing gas, e.g. a flue gas, isintroduced into the bottom of a scrubber 10 through an inlet pipe 12.The composition of a typical flue gas from a coal-fired power stationusing coal with a sulfur content of 2.46 weight percent of themoisture-free coal is as follows, in volume percent: 74.63% N₂ ; 13.98%CO₂ ; 3.30% O₂ ; 0.17% SO₂ ; and 7.92% of H₂ O. The scrubber 10 may be,for example, a conventional countercurrent or co-current packed tower,spray tower, or other conventional scrubbing apparatus, but theconventional countercurrent packed tower is preferred. The flue gasentering the scrubber may be at a higher temperature than the scrubberand may contain some residual fly ash. Water is fed through a pipe 14 toa washing spray 16 which serves to quench the gas to humidify it andreduce its temperature. Simultaneously, it cleanses the flue gas ofsolids. The latter are rejected as a slurry through a discharge pipe 18.

An aqueous scrubbing solution containing, in solution, at least tenpercent (10%) and preferably more than twenty-five percent (25%) byweight of potassium thiosulfate is continuously fed through a conduit 20into the top of the scrubber 10. The solution contains at leastsufficient potassium carbonate to react with all the SO₂ in the fluegas. By the term "carbonate" is meant either K₂ CO₃ or KHCO₃, ormixtures thereof unless otherwise expressly indicated. The scrubbingsolution also contains potassium formate and potassium sulfite.Similarly, by the term "sulfite" is meant either K₂ SO₃ or KHSO₃, ormixtures thereof unless otherwise expressly indicated. A typicalcomposition of absorbent solution introduced into the top of thescrubber through conduit 20 is approximately as follows:

    ______________________________________                                        K.sub.2 S.sub.2 O.sub.3                                                                            50.0    wt. %                                            K.sub.2 CO.sub.3 and KHCO.sub.3                                                                    0.5     "                                                KOOCH                5.0     "                                                K.sub.2 SO.sub.3 and KHSO.sub.3                                                                    1.5     "                                                K.sub.2 SO.sub.4     1.0     "                                                H.sub.2 O            balance                                                  ______________________________________                                    

The flue gas is passed upwardly in countercurrent flow to the aqueousabsorbent which enters the top of the scrubber. The temperature withinthe scrubber is maintained generally between about 120° and 180° F.,e.g. about 135° F. The principal reactions occurring in the scrubber maybe expressed by the following equations:

    K.sub.2 CO.sub.3 + SO.sub.2 = K.sub.2 SO.sub.3 + CO.sub.2  4a.

    K.sub.2 SO.sub.3 + SO.sub.2 + H.sub.2 O = 2 KHSO.sub.3     4b.

    KHCO.sub.3 + SO.sub.2 = KHSO.sub.3 + CO.sub.2              4c.

The pH of the spent effluent aqueous absorbent leaving the scrubberthrough conduit 22 is maintained between about 6.0 and 7.8 by regulatingthe amount of carbonate fed to the scrubber in relation to the quantityof SO₂ fed in the flue gas. Generally, the fresh absorbent solutionentering the scrubber through conduit 20 will be from 0.2 to 0.8 unitshigher in pH than the spent effluent absorbent solution. The range ofliquid circulation rates through conduit 22 is suitably between 2 and 20gallons/1000 CF, e.g. 10 gallons/1000 CF of gas entering the scrubberthrough line 12.

The effluent stream leaving the scrubber contains a mixture of K₂ SO₃and KHSO₃. The ratio of K₂ SO₃ to KHSO₃ increases with pH. At a pH of7.0, the molar ratio of K₂ SO₃ to KHSO₃ is approximately one. The K₂ S₂O₃ concentration remains essentially unchanged from that of the freshabsorbent solution, as does also that of the formate. Thus, thecarbonate concentration has dropped close to zero, while the sulfiteconcentration expressed as weight percent equivalent KHSO₃ has increasedto approximately 2.0 weight percent. There is a small amount of K₂ SO₄in the spent effluent absorbent.

The efficiency of absorption of SO₂ and the composition of the effluentaqueous solution are dictated by the feed rate and composition of theregenerated solution entering the scrubbing circuit through conduit 24which connects with conduit 22. This regenerated solution containsprincipally KHS, K₂ CO₃, KHCO₃ and KOOCH, along with minor amounts of K₂S, K₂ S_(x), KOH, K₂ SO₄ and K₂ S₂ O₃. The KHS along with the otherpotassium sulfides and potassium polysulfides react rapidly in a sulfiteconverter 26 with the sulfite in the spent effluent absorbent. Thereaction in the case of KHS and KHSO₃, for example, is:

    2 KHSO.sub.3 + KHS = 3/2 K.sub.2 S.sub.2 O.sub.3 +  3/2 H.sub.2 O 5.

this reaction occurs rapidly at the same or slightly higher temperaturethan that maintained in the scrubber. A sufficient residence time isprovided in the sulfite converter 26 to assure complete consumption ofthe KHS and the other sulfides. Such complete consumption is essentialto avoid evolution of H₂ S into the treated flue gas, and is assured bymaintaining an excess of KHSO₃ in the recirculating solution. Thereaction rate of sulfite conversion decreases with increasing pH. Forthis reason, the operating pH should be maintained below 8 in thesulfite converter 26, preferably between 6.7 and 7.8, e.g. 7.5. Aresidence time of 0.2 to 5 minutes is usually sufficient to provide forcomplete consumption of the KHS, and its associated sulfides, in thesulfite converter. At the same time there is, as stated, unreactedsulfite leaving the sulfite converter. The carbonate and formate passthrough the converter essentially unchanged. The concentration ofthiosulfate increases. It is this increase in thiosulfate content thatprovides feedstock for producing fresh carbonate and sulfite-reducingagent. The effluent solution is withdrawn from the converter 26 througha conduit 28. The major portion of the stream is recycled to thescrubber 10 through conduit 20. The minor portion, generally less thanten percent (10%) by volume, is pumped into the improved regenerativesection whose operation will now be described.

Regenerative Section

The general purpose of the regenerative section is to convert theincremental thiosulfate produced in the scrubbing circuit to (1)hydrogen sulfide gas, (2) the fresh carbonate required as absorbent, and(3) the KHS required for converting sulfite to thiosulfate in thesulfite converter 26. But, additionally and most importantly, theregenerative section must produce these two essential reagents in anaqueous solution with the desired value of the ratio, R, for returnthrough conduit 24 to the scrubbing circuit. The amount of hydrogensulfide gas evolved in the regenerative section correspondssubstantially in mols to the mols of SO₂ absorbed in the scrubber.

Adequate conversion of the SO₂ to K₂ S₂ O₃ via sulfite reduction in theconverter 26, without evolution of H₂ S into the treated flue gas,requires that the carbonates (K₂ CO₃ and KHCO₃) and sulfide KHS(together with minor amounts of other sulfides as previously described)in the regenerated solution fed through conduit 24 have the ratio,sometimes called the "acceptability" ratio, R, which has been previouslygenerally defined but is defined again for potassium as follows:##EQU2## where: (S⁰) = gram atoms sulfur with valence number zero/100grams solution.

(S.sup.⁻²) = gram atoms sulfur with valence number equal to -2/100 gramssolution.

ΣK = gram atoms of K/100 grams solution present in the regeneratedsolution fed to the scrubber circuit as KHS, K₂ S, K₂ S_(x), K₂ CO₃,KHCO₃, and KOH. K present in other compounds such as K₂ SO₄, KOOCH andK₂ S₂ O₃ is excluded.

The acceptability ratio, R, of the regenerated solution in returnconduit 24 should not exceed 1.0 nor be below 0.75; and is preferablywithin the range 0.75 to 0.98. If R is above 1, KHS will appear in thescrubber with consequent evolution of H₂ S. On the other hand, R mustnot be below 0.75, for in this case the sulfite concentration will buildup to an unacceptably high value with resultant formation of K₂ SO₄ inthe scrubber as a result of oxidation of the sulfite by the oxygen inthe flue gas. By maintaining R in the range of 0.75 to 1.0 inclusive,the sulfite concentration in the scrubbing circuit can be maintainedbelow 3 weight percent of the scrubbing solution and thereby avoid ahigh rate of formation of K₂ SO₄ and its subsequent crystallization fromsolution.

The manner in which my improvement achieves the foregoing objectiveswill now be described with reference to the drawing.

A small portion, e.g. about 1 percent by volume of the recirculatingsolution, is drawn off the scrubbing circuit through a conduit 30 to asurge tank 32 and thence pumped through a conduit 34 to a reductor 36.This reductor is adapted to withstand elevated temperatures andpressures and to confine a liquid phase reaction zone. The reductorvessel may be a stirred tank, a bubble column or a packed bed absorber.The withdrawn scrubber solution is subjected to reduction by a reducinggas containing CO as the principal reductant under conditions selectedto produce an effluent gas containing H₂ S and an effluent aqueoussolution whose components have an acceptability ratio, R, greater thanone and less than 1.5, e.g. about 1.1. The major portion of the H₂ S(which corresponds substantially to the sulfur absorbed in the scrubber)is produced in the reductor. The reaction is conducted in the reductionzone at a temperature between 300° and 600° F., and is preferablynon-catalytic in which case a temperature between 400° and 500° F., e.g.450° F. is preferred. The pressure is sufficient to maintain a liquidphase reaction and is generally between 10 and 100 atmospheres, e.g. 500psi. The input of gas and the input of feed solution are controlled toprovide a mol ratio of CO consumed to K₂ S₂ O₃ consumed at a value ofabout 4 to 1, although the actual amount of carbon monoxide fed to thereductor is generally in excess of the reaction requirements.

The reducing gas is introduced into the reductor 36 through a conduit37. It usually consists of a mixture of carbon monoxide, carbon dioxideand hydrogen since such a mixture is readily obtainable by partialoxidation of hydrocarbonaceous fuel. Such a partial oxidation plant isdesignated in the drawing by the numeral 38. The hydrocarbonaceous fuel,a suitable fuel oil, is introduced into the partial oxidation plantthrough a conduit 40 while the required oxygen is transferred through aconduit 42 from an oxygen source 44.

Pilot plant kinetic studies indicate that the reduction reaction isliquid-film, mass-transfer controlled at the selected reductor operatingpressure and temperature. This reaction requires large interfacial areabetween the gas and the liquid to achieve high transfer rates. The watercontent of the aqueous solution in the reductor is in the range of 30 to70 weight percent. The reductor may incorporate mechanical agitation aswell as a gas sparger.

As mentioned, the reaction in the reductor 36 is preferably carried outin the absence of a catalyst in which case the essential reducing agentis carbon monoxide since hydrogen is relatively inert undernon-catalytic conditions. Reduction of the K₂ S₂ O₃ in this fashionproduces the desired carbonates and sulfides in accordance with thefollowing simplified reactions.

    K.sub.2 S.sub.2 O.sub.3 + 4 CO + 2 H.sub.2 O = K.sub.2 CO.sub.3 +  2 H.sub.2 S + 3 CO.sub.2                                    6.

    K.sub.2 S.sub.2 O.sub.3 + 4 CO + 3 H.sub.2 O = 2 KHCO.sub.3 +  2 H.sub.2 S + 2 CO.sub.2                                              7.

    K.sub.2 S.sub.2 O.sub.3 + 4 CO + H.sub.2 O = 2 KHS + 4 CO.sub.2 8.

in addition to the foregoing products, some potassium formate isproduced according to the following reactions:

    K.sub.2 CO.sub.3 + 2 CO + H.sub.2 O = 2 KOOCH + CO.sub.2   9.

    KHCO.sub.3 + CO = KOOCH + CO.sub.2                         10.

the amount of potassium formate (KOOCH) formed reaches an equilibriumvalue which is controlled by operating with incomplete conversion of thethiosulfate, preferably between about 95 and 99 percent. The equilibriumconcentration of formate increases sharply as the thiosulfate conversionapproaches 100 percent, to reach very high values in the aqueouseffluent from the reductor. It may be desirable to have some formatepresent since its presence decreases the water vapor present over thescrubbing solution, and thus permits higher operating temperatures inthe scrubber.

The effluent gaseous product comprising H₂ S, CO₂ and unreacted CO andH₂ is withdrawn through conduit 46. The H₂ S may be converted to sulfur,for instance in a Claus plant. The other gases may be separatelyrecovered for recycle as in the case of CO and CO₂, or, after removal ofthe H₂ S, may serve as a low Btu fuel gas for reheating the SO₂ -freeflue gas.

The desired value of the acceptability ratio, R, i.e. between 1 and 1.5,preferably about 1.1, is readily achieved by proper selection of theconditions maintained in the reductor 36. The ratio of CO₂ to CO in thereducing gas fed to the reductor has a significant effect on the valueof R. The lower the ratio, the higher is the value of R. A preferredratio is about 0.5, and may be readily obtained by appropriate recycleof CO₂.

The residence time in the reductor is sufficient to assure conversion ofat least 80% by weight of thiosulfate, preferably, as stated before,between about 95% and 99%. A suitable residence time for the preferredconversion is about 1 hour.

The effluent aqueous solution having a value of R between 1 and 1.5 isconducted from the reductor 36 through a conduit 48 to a flashdecomposition zone suitably confined in a vessel 50. The finaladjustment of the acceptability ratio R to the preferred value between0.75 and 0.98 is achieved in the flash decomposition zone throughreduction in pressure by flash decomposition in this zone. The pressureis reduced to about that maintained in the scrubbing circuit. Inaddition to the evolution of dissolved H₂ S and CO₂ by the reduction ofpressure, H₂ S is also released by the decomposition reaction ofequation (3). Decomposition of the KHCO₃ also yields K₂ CO₃ (seeequation 2). An overhead condenser (not shown) maintains water refluxand overall water balance. The evolved gases may be directed through aconduit 52 to a Claus plant for conversion to sulfur.

The aqueous solution in the flash decomposition zone 50 is pumpedthrough a conduit 54 to a stirred K₂ SO₄ crystallization zone 56. Thecooling necessary to cause crystallization of the K₂ SO₄ may occur tosome extent in the flash decomposition zone 50. Generally, however, thedesired cooling is effected by cooling the crystallization zone itself.A temperature between 130° and 200° F. is low enough to precipitatesufficient K₂ SO₄ to keep the scrubbing solution clear. As more fullydescribed in an application entitled "An Improved Sulfur DioxideScrubbing System" filed on even date herewith, the solubility of K₂ SO₄is lower in the regenerated solution than in the thiosulfate-richsolution in the scrubbing circuit. The stirred slurry is conducted fromthe crystallization zone 56 through a conduit 58 to a filter systemdesignated by the numeral 60. The filter cake is washed with spentscrubbing solution withdrawn from the scrubbing circuit through aconduit 62. The sulfite in this spent scrubbing solution reacts with anyKHS and other sulfur compounds in the filter cake to form thiosulfate.Hydrogen sulfide emission from the filter system is thereby avoided andan odor-free filter cake is produced. If desired, the wash liquor may bereturned to the regenerated solution.

The filtered solution is pumped through a conduit 64 into a regeneratedsolution storage tank 66 where it is maintained under an inertatmosphere. From this tank, it is withdrawn through conduit 24 at therequired rate and fed into the sulfite converter 26 as fresh regeneratedscrubber make-up solution.

According to the provisions of the patent statutes, the principle,preferred construction and mode of operation of the invention have beenexplained and what is considered to represent its best embodiment hasbeen illustrated and described. However, it should be understood that,within the scope of the appended claims, the invention may be practicedotherwise than as specifically illustrated and described.

I claim:
 1. In a regenerative system for the removal of SO₂ from an SO₂-containing gas having a scrubbing circuit which includes a scrubbingzone and a separate sulfite conversion zone in said circuit, and aregenerative section, and wherein said scrubbing circuit there ismaintained a continuously recirculating aqueous solution which containsin solution either sodium or potassium thiosulfate in a concentration ofat least ten percent by weight of said recirculating solution, and wherein said scrubbing zone the SO₂ -containing gas is contacted undersulfite-forming conditions, with said aqueous solution which containssodium or potassium carbonate as the effective absorbent for the SO₂ toconvert said effective absorbent to the corresponding sulfite, and wherein said separate sulfite conversion zone said sulfite is converted tothe corresponding thiosulfate by reaction with the correspondinghydrosulfide and where in said regenerative section a mixture of therequired hydrosulfide and the required carbonate is produced in aregenerated aqueous solution, said regenerative section including areduction zone in which thiosulfate contained in a slipstream withdrawnfrom said scrubbing circuit is reacted with a reducing gas containing COas the effective reducing agent,the IMPROVEMENT in the regenerativesection, whereby the proper ratio of hydrosulfide to carbonate ismaintained in said regenerated solution which is returned to saidsulfite conversion zone, which inprovement comprises:
 1. conducting thereduction of thiosulfate in said reduction zone by the CO-containingreducing gas under conditions effective to produce an effluent aqueoussolution which contains sodium or potassium hydrosulfide and sodium orpotassium carbonate in admixture with other by-products in the followingratio, R: ##EQU3## where R has a value greater than 1 but less than 1.5and (S⁰) = gram atoms sulfur with valence number zero/100 gramssolution,(S.sup.⁻²) = gram atoms sulfur with valence number equal to-2/100 grams solution, and ΣM = gram atoms of M/100 grams solutionpresent in said aqueous effluent as MHS, M₂ S, M₂ S_(x), M₂ CO₃, MHCO₃,and MOH (M is either Na or K)and thereafter
 2. 2. subjecting saideffluent aqueous solution to flash decomposition in a flashdecomposition zone under conditions effective to produce saidregenerated aqueous solution having a value of R in the range of 0.75 to1 inclusive.
 2. The process according to claim 1 wherein the reductionof the value of R in said flash decomposition zone is effected byreducing the pressure maintained in said flash decomposition zone belowthat maintained in said reduction zone.
 3. The process according toclaim 2 wherein the conditions maintained in said reduction zone are atemperature between 300° and 600° F., and a pressure between 10 and 100atmospheres, and where the pressure maintained in said flashdecomposition zone is reduced to about that maintained in said scrubbingcircuit.
 4. The process according to claim 3 wherein the reduction isconducted in said reduction zone under non-catalytic conditions and thetemperature maintained in the reduction zone is between 400° and 500° F.5. The process according to claim 3 wherein the conditions maintained insaid flash decomposition zone are effective to produce an effluentaqueous solution having a value of R in the range of 0.75 to 0.98.